PR 3: Observability back-links and terminology cleanup

- Added an Observability back-link at the top of all 7 Observability-area pages (AuditLog, SystemLogs, SystemInfo, AiUsageReport, UsageQuota, BannedIps, Diagnostics/Index) consistent with the Maintenance back-link pattern from PR 2
- Renamed company-level nav label and view title from "Notification Log" to "Email & SMS Log" to distinguish it clearly from the platform-level "Platform Notifications", "Audit Log", and "System Logs" surfaces

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/SystemInfo/Index.cshtml

@@ -14,6 +14,11 @@
 </style>
 }
 
+<div class="mb-2">
+    <a asp-controller="PlatformAdmin" asp-action="Observability" class="text-muted small text-decoration-none">
+        <i class="bi bi-arrow-left me-1"></i>Observability
+    </a>
+</div>
 <div class="d-flex justify-content-end align-items-center mb-4">
     <span class="badge bg-secondary">Read-only diagnostics</span>
 </div>